[Feature] Display the model-level constraints in the documentation generated by dbt

### Is this your first time submitting a feature request?

- [X] I have read the [expectations for open source contributors](https://docs.getdbt.com/docs/contributing/oss-expectations)
- [X] I have searched the existing issues, and I could not find an existing issue for this feature
- [X] I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

### Describe the feature

When I run dbt docs generate, it fails to include constraints defined at the model level in the documentation. For instance, if I establish a primary key at the model level, this information isn't reflected in the documentation. Instead, it only appears when I define the primary key at the column constraint level. However, because I'm utilizing dbt-bigquery, I'm restricted to declaring only one primary key field at the column level. This necessitates the declaration of primary keys at the model level, thereby creating a discrepancy in the documentation.

### Describe alternatives you've considered

To enhance the documentation process in dbt for dbt-bigquery, consider implementing the ability to define multiple primary key constraints at the column level while also ensuring that primary key constraints established at the model level are accurately reported in the documentation. This improvement would streamline the documentation process and provide comprehensive information about the primary keys used in the data models.

### Who will this benefit?

_No response_

### Are you interested in contributing this feature?

_No response_

### Anything else?

_No response_